import re
import subprocess
from typing import Optional

def explain_error(error_code: str, timeout: Optional[float] = None) -> str:
    # check if the error code is a rust error code
    if not re.match(r"^E\d{4}$", error_code):
        raise ValueError(f"Invalid error code: {error_code}")
    
    try:
        explain_output = subprocess.check_output(f"rustc --explain {error_code}", shell=True, stderr=subprocess.STDOUT, universal_newlines=True, timeout=timeout)
    except subprocess.CalledProcessError as e:
        explain_output = e.output
        if isinstance(explain_output, bytes):
            explain_output = explain_output.decode("utf-8")
        return explain_output
    except subprocess.TimeoutExpired as e:
        explain_output = e.output
        if isinstance(explain_output, bytes):
            explain_output = explain_output.decode("utf-8")
        return explain_output
    if isinstance(explain_output, bytes):
        explain_output = explain_output.decode("utf-8")
    return explain_output
    